Previously on IIS6 we had a redirect in place to force all users to use a HTTPS connection for webmail on Smartermail, even though they could connect on a wildcarded domain name (ie. webmail.domain.com). Not a very elegant solution, requireing custom error messages and .asp scripts.
IIS7.5 supports the new Microsoft URL ReWrite module, and the redirect can now be implemented using the URL ReWrite plug-in in IIS Manager, and something along these lines:
Where https://securemail.domain.com is the actual SSL certificate installed on IIS